The present invention relates generally to various arrangements of optical and electronic components to form a high resolution helmet mounted display device (HMD).
Helmet mounted display devices (HMD""s) are rapidly becoming the standard display device for virtual reality and xe2x80x9cTeleprescencexe2x80x9d applications. Such devices generally consist of one or more compact image displaying devices mounted on a helmet type frame that the viewer wears on their head. The said image displaying devices project images into the viewer""s eyes via a series of lenses or mirrors so that the viewer perceives the image or images to originate from a source outside of the helmet. In the case of stereoscopic HMD""s a separate image is presented to each of the viewer""s eyes so that a three dimensional (3D) image can be formed. This 3D image has the additional reality of 3D depth cues such as stereo parallax (the differential shifting of objects within the image due to varying distance from the camera or other imaging source)
In addition to these depth cues, the viewer""s perception that the xe2x80x9cvirtualxe2x80x9d or xe2x80x9csyntheticxe2x80x9d world that is being presented by virtue of the HMD can be further enhanced by incorporating a tracking system on the HMD so that as the viewer moves their head (pitch, roll or yaw) the projected image moves in a corresponding manner such that it is consistent with the formation of images that would have been viewed had the viewer been moving in a similar way in the real world. The position of the viewer within the virtual world in terms of X,Y,Z (spatial coordinates) is also significant and is often utilized to change the projected images to be consistent with the viewer moving through the virtual world. This type of movement is generally not controlled directly by tracking systems of the viewer or operator but more generally by virtue of a joystick, data glove, cyberpuck or other spatial positioning device.
At present, the preferred HMD display technology is utilizing compact color LCD (liquid crystal display) panels. However monochrome LCD panels and monochrome and color CRT (cathode ray tube) type displays have also been used to good effect.
The most important factors in the choice of the type of display technology chosen for an HMD is image quality and the compactness of the display.
As HMD""s generally have very wide viewing angles (the angle subtended from the corners of the image to the pupil of the viewer""s eye) the image resolution becomes a very important factor. Currently, CRT technologies (particularly monochrome CRT""s) offer the highest resolutions. Unfortunately they tend to be rather bulky devices and require high voltages and have a relatively high power consumption. It is possible to optically combine several monochrome display devices to form a full color image. Such techniques are well established and are known to those experienced in the field. Utilizing such techniques it is possible to achieve high resolutions and full color, however bulkiness and weight of the resulting display device is often too great for a helmet mounted application.
LCD panels overcome many of these problems by virtue of their compact size and low power consumption. However, they traditionally have several disadvantages from CRT""s. The first of these is that their resolution is significantly lower than state of the art CRT""s and their color saturation is also significantly less than the corresponding CRT type display. This results in a reduced color space (as shown in FIG. 37). Traditionally LCD""s have also had a significantly slower response time (time taken for a pixel to change from 10% to 90% brightness or vice versa), which has been a problem for use with rapidly changing images. However, the newer LCD technologies such as TFT (thin film transistor) and dual active scan LCDs have effectively addressed these problems.
The most significant single problem now facing designers of HMDs is that of resolution. Most current low-medium performance HMD""s utilize 0.7xe2x80x3 color LCD""s. These offer a resolution of approximately 180,000 pixels (red, green, and blue pixels counted separately). This means that the display is capable of approximately 60,000 color picture elements. HMD""s of this quality display relatively poor images and result in a significantly diminished virtual reality (VR) effect. Improved technology in recent years is resulting in a new generation of compact high resolution LCD panels that offer significant advantages over these earlier designs. It is now possible to purchase compact color LCD panels with resolutions as high as 640*480 (307,200) color triads (color picture elements). This is equivalent to a pixel count of approximately 920,000. However, at present these displays are very expensive, resulting in HMD""s utilizing these LCD panels to be priced well outside the general computer/gaming market.
Even at these resolutions the viewing angle is often still reasonably limited, which results in a reduced visual impact of the HMD. Other solutions to this problem which have been attempted are optical fibre display""s and direct retinal scanning. The first approach utilizes a bunch of optical fibers to optically couple a high resolution image from a relatively bulky remote image source to the HMD. This makes possible a display with a color pixel count in the 1,000,000 region whilst retaining a lightweight HMD. Unfortunately, this type of display tends to be extremely expensive and, although technically feasible, is priced way out of the general computer/gaming market. The second approach is very new and at the present time would seem to be still at the theoretical/early prototype stage. The basic approach is to scan a colored beam of light (probably consisting of three co-axial beams of red, green, and blue light) directly onto the retina of the viewer""s eye thereby rendering an image. The approach seems to have many merits. The first and foremost being that it is theoretically possible to achieve high resolutions. In addition, if micro LASERs or LEDs are used as the light source then (by virtue of their high color purity) a much enhanced color space is achievable. Theoretically, the color space of such a display could be significantly superior to that of a CRT type display. Although theoretically attractive, this approach has several major technical hurdles that have to be overcome for the device to become a commercial success. It would appear that, at this stage, these technical hurdles are the generation of sufficiently compact light sources that can produce a collimated full color scanable light beam and the generation of a very compact high speed optical scanning apparatus for scanning the said beam of light across the viewers retina.
An alternative approach that I have developed is a xe2x80x9cmid-groundxe2x80x9d between the two approaches. It is possible to utilize a micro-mirror device such as those produced by Texas instruments to direct monochrome light from a compact source through a series of lenses, mirrors, or a combination of both, to the eyes of a viewer to produce a high resolution color image. Further, it is possible by changing the color of the monochrome light and by projecting the image as a sequence of the red, green, and blue (or other acceptable optical primaries such as orange, green, and blue) components of the image to produce a full color image with a color resolution equivalent to the resolution of the digital micro-mirror device (DMD) i.e. the apparent pixel resolution will be three times as great as the actual resolution of the DMD chip. This configuration has all of the advantages of both of the previous HMD display technologies. Firstly, the optical system is relatively simple and requires none of the complex active components that comprise the scanning device of the direct retinal display. However, the DMD technology can offer significantly higher resolutions than the LCD technology. DMD chips have been fabricated with resolutions close to the 2,000,000 pixel mark, which would result in an equivalent HMD resolution of 6,000,000 pixels per eye. The DMD chips are very compact and readily lend themselves to incorporation into a HMD shell. In addition, the purity of the light source bounced off the DMD is entirely independent of the DMD chip. Thus, it is possible by using LED or LASER light sources to achieve an optimal color space that could easily surpass CRT type displays (see FIG. 37).
There are many reasons why this design is the preferred implementation. Firstly, it offers the best features of all previous HMD technologies. Secondly, it is proven technology and thirdly by virtue of the DMD fabrication techniques, it lends itself effectively towards the VR (virtual reality) environment. An example of this is shown in FIG. 35. As can be seen, it is possible to produce several DMD arrays on a single chip. Ordinarily, this configuration would be unacceptable for video projection as it would produce dark lines at the interstices of the DMD arrays. However, in the case of VR, the surrounding display regions fall into the peripheral vision part of the viewer""s eye""s and thus the viewer is relatively insensitive to the image discontinuity.